For shading vehicle windows, it is known to use roller-blind arrangements. The roller blinds have a wind-up shaft which is mounted in the vehicle and to which the shade material is attached with one edge. With the aid of the drive device, the shade material is unwound from the wind-up shaft. Various gear devices are known for the drive.
For a different group of window roller blinds, the unwinding of the shade material is effected manually, in that a handle attached to the front edge of the shade material is grasped and guided away from the wind-up shaft. Here, the front edge of the shade material is understood to be the edge that extends the greatest distance in the pulling-out motion.
The roller blind is typically wound with the aid of a spring motor. For the spring motor, two types are also known in the art. According to one type, a helical spring is contained in a wind-up shaft having a tubular construction. The helical spring is fixed at one end to the chassis and is locked in rotation at the other end to the wind-up shaft. The spring motor is wound when the shade material is pulled out. In this way, elastic energy is stored, which is converted into movement when the shade material is rewound onto the wind-up shaft. The winding or unwinding requires about 10 revolutions of the wind-up shaft.
In addition to the use of a helical spring as the spring motor, spring motors based on flat spiral springs are also known. Flat spiral springs have the advantage that the biasing force is nearly constant over the stroke of the spring motor, i.e., corresponding to the length of extension of the shade material. Accordingly, a spring motor based on a flat spiral spring does not need to have a significantly longer stroke than what corresponds to the required number of revolutions of the wind-up shaft during extension or retraction of the shade material. It is sufficient if the flat spiral spring motor merely is designed with an overstroke corresponding to the necessary tolerance. The flat spiral spring motor, moreover, also is relatively rattle free from vehicle vibrations.
However, when unwinding or winding, the flat spiral spring motor can exhibit an unpleasant property in that the spring windings, during continuous motion of the spring motor, generate radial vibrations in the transition from one winding to the other winding which lead to rattling noises. Two windings of the flat spiral spring motor are produced from that part contacting the inside of the spring housing and from the other part that is already wound on the shaft of the spring motor as a winding.